Solid-state lighting (SSL) is a type of lighting that does not use an electrical filament or a gas in the production of light. A primary advantage of SSL over conventional lighting technologies is the potential energy savings as a result of its higher luminous efficiencies over conventional lighting devices. For example, SSL is capable of 50% efficiency with 200 lumen per watt (lm/W) efficacy (compared to 15 lm/W for incandescents and 60-90 lm/W for fluorescents) and up to 100 khr lifetimes. This is approximately 100 times the lifetime of conventional incandescent bulbs and 10 times the lifetime of fluorescents. The Department of Energy (DOE) has set a goal of 50% electrical-to-optical system efficiency with a spectrum accurately reproducing the solar spectrum by 2020. The Optoelectronic Industry Development Association (OIDA) aims for 200-hm/W luminous efficiency with a color rendering index greater than 80.
Each of these conventional methods and devices has deficiencies. Color mixing is hindered by the absence of an efficient LED material in the 500 nm to 580 nm (green to-yellow) range. Wavelength conversion suffers from phosphor conversion loss and package designs that do not extract phosphor-converted light efficiently.
SSL devices primarily include light emitting diodes (LEDs), which include a small chip semiconductor, i.e. the LED source, mounted in a reflector cup on a lead frame. The LED source generates photons of light at a first wavelength when energized. The reflector cup reflects photons out of the LED. An optic, generally a silicone or epoxy encapsulation, aids in light extraction from the LED source and protects the LED components.
High efficiency generation of white light with LEDs has conventionally been according to one of three methods: 1) color mixing; 2) wavelength conversion; or 3) a combination of methods 1 and 2. Color mixing is the use of multiple LEDs across the visible spectrum (e.g. blue+green+red LEDs), which combine to produce a white light. Wavelength conversion is the use of a single, efficient, short wavelength LED emitting light at the first wavelength, which is then at least partially absorbed by a phosphor within the LED and re-emitted at a second wavelength. LEDs under method 2 are generally referred to as phosphor-converted LEDs (pcLEDs).
Conventional pcLEDs have generally two structural arrangements. First, the phosphor can encompass the LED source of the LED. The phosphor is typically a YAG:Ce crystalline powder in direct contact with the blue wavelength emitting LED source. Both are positioned upon a heat sink base and surrounded by an optic. The other arrangement is a scattered photon extraction (SPE) pcLED, which positions a planar phosphor-layer at a distance away from the LED source. Herein, the YAG:Ce phosphor, in powder form, creates a diffuse, semitransparent layer upon an acrylic optic with a planar surface.
When the phosphor is in direct contact with the LED source, the phosphor suffers from optical losses by reflection of phosphor-emission back into the LED source rather than through the optic and out of the LED. This can account for up to 60% of the total phosphor emission. The SPE pcLED suffers from scattering of the phosphor emissions. Scattering is the result of substantial differences in the indices of refraction of the phosphor powder and the material that encapsulates the phosphor (air, silicon, PMMA, or glass). The index of refraction, n, is a measure of the relative speed of light in a medium as compared to in a vacuum (where nvac=1). When light passes from one medium to another medium with a substantially different index of refraction, the speed and direction of the light changes and is known as refraction. Refraction can lead to a randomization, or scattering, of the directionality of the light. Scattering then reduces efficiency by increasing the path length (a) inside the phosphor layer by trapping of the emissions by total internal reflection and (b) inside the device package because of random directionality of the phosphor emission, both of which can lead to reabsorption and optical loss.
These phosphor-related deficiencies are then compounded by secondary losses encountered by other package design deficiencies, such as imperfections of the reflector cup within the LED. While the reflector cup is intended to direct the phosphor-emission out of the LED, internal reflections and path randomization can trap a portion of the phosphor-emission, such as between the reflector cup and the phosphor, and decrease LED efficiency by approximately 30%.
Thus, to reach the efficiency goals set forth by the DOE, the problems associated with package design must be eliminated by designing a high efficiency LED that resolves the issues identified above.